What engine did the tractor have originally? I bet that if it was a briggs, any similarly sized briggs will work. Same with Tecumseh. I think the brake for them was 3 - 11 HP, and then 11+. But some 11 HP Tecumseh's were really 13's that were just labeled because it was cheaper to use existing engine's.
Worst case, you could redrill the chassis to fit the bolt pattern of the donor engine. Just center the engine good so the pulley is in about the same spot.